In copper plating from a Cu2+-containing solution, which half-reaction occurs at the cathode?

• A. Cu -> Cu2+ + 2 e-
• B. Cu2+ + 2 e- -> Cu(s)
• C. 2 I- -> I2 + 2 e-
• D. Na+ + e- -> Na(s)

Answer: (B)

In electroplating, the cathode is where reduction occurs, so copper ions in the solution gain electrons and deposit as solid copper on the surface. The appropriate half-reaction is Cu2+ + 2 e- → Cu(s). This is why a copper coating forms on the object being plated.

The other possibilities would involve oxidation or would not occur under typical aqueous plating conditions. Oxidation of copper to Cu2+ happens at the anode, not the cathode. Oxidation of iodide to I2 is also an anodic process. Reduction of Na+ to Na(s) would require conditions that aren’t compatible with an aqueous copper-plating bath (water would be reduced to H2 well before sodium metal could form).

So the cathodic process you expect here is the reduction of copper ions to metallic copper.